The present invention relates to a controller for a rechargeable battery, a method for estimating the temperature of a rechargeable battery, and a method for determining deterioration of a rechargeable battery with the temperature estimation method.
In recent years, rechargeable batteries have been combined with fuel cells, solar cells, or power generators to form power supply systems. A power generator is driven by natural power, such as wind power or water power, or by artificial power, such as power generated by an internal combustion engine. A power supply system using a rechargeable battery stores excess power in the rechargeable battery to improve energy efficiency.
An example of a power supply system is a hybrid electric vehicle (HEV) using an engine and a motor as its power sources. When the engine outputs more power than necessary to drive the vehicle, the HEV drives its generator using the excessive power to charge the rechargeable battery. When the vehicle is braking or decelerating, the HEV drives the motor with the vehicle wheels and charges the rechargeable battery using the motor as a power generator. When the engine outputs less power than necessary, the HEV compensates for the lack of power by discharging the rechargeable battery and driving the motor.
In this way, the HEV accumulates energy in the rechargeable battery. Conventional automobiles release such energy into the atmosphere as heat. The energy efficiency of an HEV is higher than the energy efficiency of a conventional automobile. Thus, the HEV greatly improves fuel efficiency as compared with conventional automobiles.
The electrochemical reaction of the rechargeable battery occurring when the rechargeable battery is charged or discharged is dependent on the temperature of the rechargeable battery. In the power supply system, the rechargeable battery, which is typically formed by a plurality of cells connected in series, generates a large amount of heat. Further, the rechargeable battery may have its performance lowered or its life shortened when the temperature of the rechargeable battery exceeds a predetermined temperature. Japanese Laid-Open Patent Publication Nos. 2001-313092 and 2001-86601 describe examples of techniques for adjusting the temperature of a rechargeable battery with a cooling fan.
Further, the battery temperature must be accurately obtained in a power supply system to efficiently operate the cooling fan or determine abnormalities in the cooling fan. With the techniques described in the above publications, the temperature of the rechargeable battery is obtained using a battery temperature measured by a temperature sensor and a battery temperature obtained through calculation. The calculation of the battery temperature is based on the amount of Joule heat generated when the rechargeable battery is charged or discharged and the cooling capability of the cooling fan.
However, the rechargeable battery actually involves not only generation of Joule heat but also other types of heat. Thus, the accuracy of the battery temperature calculated through the conventional techniques is low. As a result, the temperature of the rechargeable battery obtained through the conventional techniques may be inaccurate. A highly accurate temperature sensor may be used to solve this problem. However, the cost of such a highly accurate temperature sensor is high.
Initial deterioration of the rechargeable battery is also detectable if the accurate battery temperature of the rechargeable battery is obtained through calculation and compared with the battery temperature measured by the temperature sensor. In this way, it is important to obtain accurate battery temperature of the rechargeable battery through calculation.